Individual gp160 clones were screened for infectivity as Env-pseudotyped viruses in a luciferase reporter gene assay in JC53-BL (TZM-bl) cells. Functional env clones were sequenced and their neutralization phenotypes characterized by using soluble CD4, monoclonal antibodies and serum samples from infected individuals and non-infected recipients of a recombinant gp120 vaccine. Env clones from geographically diverse HIV-1 isolates were selected that were not unusually sensitive or resistant to neutralization and comprised a wide spectrum of genetic, antigenic and geographic diversity 3. These reference reagents will facilitate proficiency testing and other validation efforts aimed at improving assay performance across laboratories and can be used for standardized assessments of vaccine elicited neutralizing antibodies. Using the reference virus reagents described above, the Humoral immunology Core Section has screened hundreds of animal sera generated by investigators at the VRC.